€543 HYDROGRAPHIC MANUAL PaGE 610 



if separate units are used. Disconnect the hydrophone cable from the input of the 

 sono-radio buoy amplifier hnd connect the output of the attenuator across these termi- 

 nals. Remove the keying tube and connect the rectifier voltmeter across the terminals 

 supplying voltage for this tube. The leads to the input and output should be as short 

 as possible and in some cases should be shielded. All equipment should be grounded. 

 A warm-up period of a half hour should be allowed before making measurements. 



The following measurements should be made for each sono-radio buoy after its 

 optimum gain has been determined in accordance with 654(1): The oscillator fre- 

 quency is adjusted to some selected value between 50 and 200 cycles or, if the amplifier 

 is resonant at some frequency, the oscillator is adjusted to this resonant frequency. 

 The gain control of the sono-radio buoy is set at the optimum value which has been 

 determined. The attenuator is then adjusted to make the output voltage of the ampli- 

 fier read some preselected reference value. This reference value should be used for all 

 future measurements of all sono-radio buoys of this type. The reference voltage should 

 be some value between 10 and 25 volts, or a value below amplifier saturation, so that a 

 change of input voltage produces a proportional change of output voltage of the ampli- 

 fier. The ratio of output to input voltage is now noted and recorded. It is this ratio 

 that should be used for adjusting the gain of the sono-radio buoy in the future. 



Practically the same procedure is followed in adjusting the gain just before the 

 sono-radio buoy is put on station. In this case the output voltage is set at reference 

 value and the gain control of the sono-radio buoy is adjusted until the ratio of output 

 to input voltages equals the value previously measured and recorded. 



The apparatus used for direct measurements may be a permanent part of the radio- 

 room testing equipment or it may be portable. The advantage of using portable instru- 

 ments is that in certain cases it is not necessary to remove the sono-radio buoy equip- 

 ment from the drum to make the gain adjustments and measurements. The direct 

 measurement method is the most positive method now used, although it is not com- 

 pletely satisfactory as it does not take into account the characteristics of the hydrophone, 

 keying circuit, or radio transmitter. Changes in the characteristics of any of these 

 parts may affect the performance of the sono-radio buoy, and unless the amount of 

 change is known, it cannot be compensated for by the direct measurement method. 

 Another undersirable feature of this method is that the measurements require consider- 

 able time and' care. 



A variation of the direct measurement method, wliich has the advantage of being 

 simpler and quicker to perform, may be used to adjust a sono-radio buoy, although the 

 results are not quite so positive. The apparatus for gain measurements should consist of 

 an oscillator and attenuator similar to those described in 6542, except that they should be 

 portable. It is not necessary to remove the electric equipment from the buoy — 

 merely disconnect the hydrophone andconnect the attenuator in its place. From pre- 

 vious tests as described in 654(1) the value of the input voltage to the audio amplifier 

 at optimum gain, for satisfactory operation, is known. Adjust the test-circuit attenu- 

 ator until this value of voltage is across the input of the amplifier. Starting with the 

 gain control of the amplifier at a low value, increase the gain slowly until the radio 

 transmitter just starts to operate. Repeat this operation several times until the posi- 

 tion of the gain control has been accurately determined for the point of transmitter 

 operation. 



The point of operation of the transmitter can be established by means of some 

 indicating device coupled to the antenna circuit. Some of these devices are described 



